The present invention relates to an audio system mounted on a motor vehicle, and more particularly to an isolator circuit provided in the system for eliminating noise entering the system.
FIG. 6 shows an electric circuit of a conventional sound-reproduction system such as a CD player mounted on a motor vehicle. In the system, a signal source A is connected to an amplifier B through shielding wires L1 and L2. If an impedance exists in the ground wire L2, currents dependent on various types of noise radiated from the alternator, the ignition system and the horn of the motor vehicle enter the ground wire L2. As a result noise voltage en enters into an output signal (source signal) of the signal source A in series between an output EOUT of the signal source A and an input EIN of the amplifier B. The noise voltage en is amplified by the amplifier B which results in the production of unwanted sound from a loudspeaker (not shown).
In order to eliminate such noise, the conventional sound-reproduction system is provided with an isolator circuit as shown in FIG. 7 between the output EOUT and the input EIN of the signal source A and the amplifier B.
The isolator circuit includes an operational amplifier 1, capacitors C1, C2 and C4 and resistors R1, R2, R3 and R4. An output terminal E1 of the signal source A is connected to an inverting input of the operational amplifier 1 through the capacitor C1 and the resistor R1. Another output terminal E2 of the signal source A is connected to a noninverting input of the operational amplifier 1 through the capacitor C2 and the resistor R3. An output of the operational amplifier 1 is connected to an input terminal E3 of the amplifier B and to the inverting input of the amplifier 1 through the resistor R2. Between the resistor R3 and the noninverting input of the operational amplifier 1, another input terminal E4 of the amplifier B is connected through the capacitor C4 and the resistor R4. The input terminal E4 is connected to the ground. Resistance R5 has a large valve as a reference voltage.
In such an isolator circuit, if a noise voltage en1 is produced between terminals E5 and E6 of the grounds of the signal source A and the amplifier B, the noise voltage en1 is added to a voltage en2 of an audio signal of the signal source A, if the ground terminal E5 is regarded as a reference. Thus, the voltage en1+en2 is applied to the inverting input of the operational amplifier 1.
If the impedance of the audio signal source of voltage en2 is sufficiently small, gain G1 of the operational amplifier 1 as viewed from the inverting input is represented as EQU G1=R2 / R1
Consequently, the output Op1 of the operational amplifier 1 is EQU Op1=-( en1+en2 )
Thus, the noise is represented as -en1.multidot.R2/R1.
On the other hand, gain G2 of the operational amplifier 1 as viewed from the noninverting input is represented as EQU G2=( R1+R2 ) / R1
Since the noise voltage en1 is divided by resistors R3 and R4 and applied to the noninverting input, the output Op2 of the operational amplifier is represented as EQU Op2=en1.multidot.( R1+R2 ) / R1.multidot.R4 / (R3+R4)
In order to cancel the noise voltage en1 from the output of the operational amplifier 1, the output Op2 must be equal to the noise -en1.multidot.R2/R1 of the output Op1.
Consequently, EQU -en1.multidot.R2 / R1=en1.multidot.( R1+R2 )/ R1.multidot.R4/( R3+R4 ), EQU R2 / R1=R3 / R4
Thus, the common-mode noises caused by the alternator, ignition system, horn and others are suppressed.
FIGS. 8 and 9 show examples of the conventional isolator circuit. In the isolator circuit of FIG. 8, the output signal from the signal source A is applied to the inverting input of the operational amplifier 1. In FIG. 9, the output signal is applied to the noninverting input thereof. FIG. 10 is a graph of frequency responses of these circuits, in which the line a represents the characteristic of FIG. 8 and the line b shows the characteristic of FIG. 9.
The cutoff frequency of the circuit of FIG. 8 is determined by resistors and capacitors. The cutoff frequency of the circuit of FIG. 9 is determined by the product of resistors and capacitors. The cutoff frequency is frequency below which the output becomes smaller than -3dB.
Namely, the cutoff frequency of the line a is about 100 Hz and the cutoff frequency of the line b is about 200 Hz. As seen from the graph, the characteristics of both circuits are inferior in a low frequency range. In other words, the circuits can not remove noises in the low frequency range.